With the advent of more fuel efficient automobiles and the desire to improve quality, the automotive industry has become increasingly interested in improvement of wheel alignment measurement systems. In particular, the desire to provide wheel alignment measurement systems that are highly accurate yet utilize minimal time to operate and require minimal maintenance has risen in importance.
Conventional wheel alignment measurement systems utilize various direct contact measuring devices. These systems use conventional tools or adapters which locate to the wheels or spindles or other features of the rotating tire or wheel assembly and often rely on empirical or interpretative evaluation to predict alignment values. These systems are cumbersome, bulky and time consuming to operate. They also suffer from various problems associated with mechanical devices. The problems are often related to the difficulty in obtaining accurate measurements due to the variations in tire and/or wheel size, variations in tire type and inflation, variations in vehicle weight and wheel base, and variations in vehicle drive method and placement of the inspection apparatus. In addition, wear of the testing system apparatus over extended periods of use and distortions of the actual tire profile caused by the direct contact of the measuring tools adds to the unreliability and inaccuracy of the present direct measurement devices.
In addition, wheel alignment systems should also be utilized in the continuous preventative maintenance programs of automotive vehicles on the road. It is known that vehicle wheel alignment is detrimentally effected by collisions with other vehicles, contact with street curbs or speed bumps, and travel over pot holes. As such, a wheel alignment measurement system should allow for recalibration and realignment of the wheels to the initial alignment specifications determined in the manufacturing process. Thus, the preventative maintenance provider or service station should also have an accurate alignment measurement system which is capable of measuring wheel alignment to assist in bringing the wheel alignment back to the original manufacturing specifications.
U.S. Pat. No. 4,745,469 issued to Waldecker and U.S. Pat. No. 4,899,218 also issued to Waldecker, both disclose the use of a non-contact, vehicle wheel alignment apparatus which uses a structured light source. More specifically, the Waldecker systems utilize an optically-fanned laser light source to project at least two structured light beams onto one tire while the tire wheel assembly is being rotated. Contour lines are formed by these fanned laser light sources and are projected onto the tire and are thereby read by a video camera with an associated sensor module. The video camera takes multiple optical pictures of readings which are subsequently interpreted by the sensor modules. The sensor modules are offset from the optical plane of the structured light sources to allow triangulation with respect to the tire and laser light source to determine the spatial position of the contour lines.
Similar to geometric alignment measuring methods, the Waldecker system measures relative locations of two or three points on a tire sidewall to determine an indirect rotational plane. As the tire rotates, sets of measurements are taken several times per rotation. The measurements are then mathematically manipulated in an attempt to eliminate errors caused by various phenomena such as run-out, varying tire pressure, offset between the measured plane and the true alignment plane, etc. The Waldecker system does not track a single point fixed in reference to the true spindle axis throughout a single wheel rotation or directly measure, in a common coordinate system, the position and/or angles of the rotational planes of the wheels to be tested.
U.S. Pat. No. 5,048,954 issued to Madey discloses a laser-based wheel alignment measurement system. The Madey wheel alignment system utilizes a low power visible laser which is split into two generally parallel beams by a partially transmitting mirror and a series of plane front surface mirrors positioned in front and alongside the vehicle being serviced. The two beams are reflected from a set of mirrors attached to the rims of the particular vehicle wheels that are being aligned. The vertical and horizontal angles of each of the wheel-mounted mirrors are set by calibrated lead screws to cancel the angular displacement of the wheels expected when the wheels are properly aligned. The reflected beams are imaged through a large aperture beam combiner on a common viewing screen. Required tow-in and camber measurements are indicated when the laser beams reflected from the wheel-mounted mirrors overlap at the center of the viewing screen.